The conversion of a gaseous or liquid reformable fuel to a hydrogen-rich carbon monoxide-containing gas mixture, a product commonly referred to as “synthesis gas” or “syngas.” can be carried out in accordance with any of such well known fuel reforming operations such as steam reforming, dry reforming, autothermal reforming, and catalytic partial oxidation (CPOX) reforming. Each of these fuel reforming operations has its distinctive chemistry and requirements and each is marked by its advantages and disadvantages relative to the others.
The development of improved fuel reformers, fuel reformer components, and reforming processes continues to be the focus of considerable research due to the potential of fuel cells, i.e., devices for the electrochemical conversion of electrochemically oxidizable fuels such hydrogen, mixtures of hydrogen and carbon monoxide, and the like, to electricity, to play a greatly expanded role for general applications including main power units (MPUs) and auxiliary power units (APUs). Fuel cells also can be used for specialized applications, for example, as on-board electrical generating devices for electric vehicles, backup power sources for residential-use devices, main power sources for leisure-use, outdoor and other power-consuming devices in out-of-grid locations, and lighter weight, higher power density, ambient temperature-independent replacements for portable battery packs.
Because large scale, economic production of hydrogen, intrastructure required for its distribution, and practical means for its storage (especially as a transportation fuel) widely are believed to be a long way off, much current research and development has been directed to improving both fuel reformers as sources of electrochemically oxidizable fuels, notably mixtures of hydrogen and carbon monoxide, and fuel cell assemblies, commonly referred to as fuel cell “stacks,” as convertors of such fuels to electricity, and the integration of fuel reformers and fuel cells into more compact, reliable and efficient devices for the production of electrical energy.
CPOX reforming, or simply CPOX, has attracted particular attention as a way of supplying hydrogen-rich reformate to fuel cell stacks, for example, those having nominal power ratings of anywhere from 100 watts to 100 kilowatts, and all power ratings in between. Among the advantages of CPOX reforming is that the reaction is exothermic in contrast to steam reforming and dry reforming which are endothermic reactions that require an external source of heat.
Furthermore, CPOX reactions are generally faster than other reforming reactions which allows for the construction of relatively small reformers capable of fast start-up and rapid response to changes in load. CPOX reformers also tend to be simpler in design than reformers that require the handling of water and steam, for example, steam reformers and autothermal reformers, which require storage units for water, heating units for the production of steam, burner or combustion units for supplying heat to drive endothermic reforming reactions, and the like, and their associated fluid routing and operation-monitoring and control devices.
However, and as previously recognized (see, e.g., U.S. Pat. Nos. 6,790,431 and 7,578,861), the typically high levels of heat produced during CPOX reactions can have undesirable consequences including damage to the reformer and/or components thereof such as the CPOX catalyst, catalyst support, and other structural components. This is a major drawback of many current CPOX reformer designs and one in need of an effective solution.
One known type of CPOX reformer includes a catalyst support component, commonly referred to as a “catalyst monolith,” “monolith catalyst support,” “monolith substrate.” or simply a “monolith” which has a CPOX catalyst or catalyst system deposited thereon.
Monoliths can be classified on the basis of two general configurations: a first configuration characterized by a metal or ceramic body of honeycomb-like, channeled, metallic gauze or spiral-wound corrugated sheet structure presenting an essentially linear gaseous flow path therethrough, and a second configuration characterized by a metal or ceramic foam body of reticulated, or open, pore structure presenting a tortuous gaseous flow path therethrough. Representative monoliths of one or the other general type are disclosed in, for example, U.S. Pat. Nos. 5,527,631; 6,402,989; 6,458,334; 6,692,707; 6,770,106; 6,887,456; 6,984,371; 7,090,826; 7,118,717; 7,232,352; 7,909,826; 7,976,787; 8,323,365; and, U.S. Patent Application Publication No. 2013/0028815.
As shown in FIG. 1A, monolith 100, which is of a common prior art type, viewed in longitudinal cross section includes a honeycomb-like ceramic body 101 made up of numerous channels 102 impregnated or wash-coated with CPOX catalyst, an inlet end 103 for admitting a gaseous CPOX reaction mixture, i.e., a mixture of a gaseous oxidizing agent, typically air, and reformable fuel, e.g., a gaseous fuel such as methane, natural gas, propane or butane or a vaporized liquid fuel such as gasoline, kerosene, jet fuel or diesel, an outlet end 104 for the discharge of hydrogen-rich, carbon monoxide-containing reformate product (syngas) and a CPOX reaction zone 105 which is essentially coextensive with the entire monolith.
CPOX reaction zone 105 can be considered as having an inner, or central, region 106 through which a corresponding inner, or central, portion of a gaseous CPOX reaction mixture stream inherently flows within a relatively high range of velocity V1 surrounded by an outer, or peripheral, region 107 through which a corresponding outer, or peripheral, portion of the gaseous CPOX reaction mixture stream inherently flows within a relatively low range of velocity V2.
Monoliths typically experience fairly high CPOX reaction temperatures, for example, on the order of from 600° C. to 1,100° C. In the case of honeycomb-like monolith 100, these high temperatures, coupled with the inherent differential in flow velocities V1 and V2 of the CPOX reaction mixture stream flowing within inner and outer regions 106 and 107, respectively, of CPOX reaction zone 105 tend to account for the observed operational drawbacks of monolith 100 and other essentially linear flow path monoliths where CPOX reforming is concerned.
At CPOX reaction temperatures of 600° C.-1,100° C., monolith 100 radiates a good deal of heat at its inlet end 103. Even with careful monitoring and control of the CPOX reaction conditions, it can be difficult to prevent or inhibit the phenomenon of “flashing,” i.e., the premature combustion of CPOX gaseous reaction mixture stream within radiant heat zone 108 as the stream approaches inlet end 103. Heat of exotherm of the CPOX reaction occurring within initial CPOX reaction zone 109 proximate to inlet end 103 radiates outwardly therefrom into radiant heat zone 108. This radiant heat can be of sufficient intensity to raise the temperature of the advancing CPOX reaction mixture stream (indicated by the arrows) to its flash point. Flashing of the CPOX reaction mixture within radiant heat zone 108 causes undesirable thermal events, raising the temperature to a point where catalyst can be vaporized or deactivated and/or reformer structure can be damaged or rendered inoperative. These thermal events can also lead to cracking of fuel within this zone and, consequently, increased coke (carbon particle) formation resulting in deterioration of CPOX catalyst performance. Where the hydrogen-rich reformate effluent is utilized as fuel for a fuel cell stack, coke and unreformed higher hydrocarbon fragments contained therein will also deposit upon the anode surfaces of the fuel cells resulting in reduced conversion of product reformate to electricity.
As further shown in FIG. 1A, the aforementioned differential in flow velocities V1 and V2 of the CPOX reaction mixture stream within, respectively, inner and outer regions 106 and 107 of CPOX reaction zone 105 are also primarily responsible for the differential in CPOX reaction temperature ranges T1 and T2 in these regions. Thus, the higher velocity V1 of the CPOX reaction mixture stream within inner region 106 results in a higher rate of CPOX reaction therein and an accompanying higher reaction temperature T1 and, conversely, the lower velocity V2 of the CPOX reaction mixture stream within outer region 107 results in a lower rate of CPOX reaction therein and an accompanying lower reaction temperature T2. The temperature profile across inner and outer regions 106 and 107 can be represented by temperature curve 110. A sharp rise in CPOX reaction temperature T1, if high enough, can result in damage to, and even total destruction of, monolith 100.
As shown in FIG. 1B, prior art-type foam monolith 150 viewed in longitudinal cross section includes a ceramic foam body 151 characterized by a reticulated, or open, network of interconnected pores and pore channels 152 supporting a CPOX catalyst or catalyst system deposited thereon by conventional or otherwise known procedures. e.g., impregnation or wash coating.
One drawback of foam monoliths of all types is their higher pressure drops due to their higher resistance to flow compared with linear-flow monoliths such as honeycomb-like monolith 100 of FIG. 1A. Higher pressure drops require higher operational pressures, and therefore higher energy consumption, to meet target flows. Another inherent drawback of foam monoliths lies in the nature of the flow paths of gaseous reactants and reaction products therein (as indicated by the arrows). The characteristic randomness of these flow paths results in very uneven temperature profiles within the monolith (e.g., as indicated by temperature curve 153), increasing the risk of thermal shock due to hot spots and/or reduced CPOX conversion rates due to cold spots.
Foam monoliths of all types are also susceptible to flashing much as in the case of the linear flow path monoliths discussed above. In addition, foam monoliths are prone to other drawbacks that are characteristic of their kind. Depending on the way in which known and conventional foam monoliths are manufactured, they can possess a relatively fragile pore network, especially within their central regions, or they can possess a more robust pore structure throughout. Both types of foam monolith are subject to disadvantages.
In the case of foam monoliths possessing a relatively fragile core region, thermal shock resulting from rapid thermal cycling of the CPOX reformer (typical of CPOX reformers that supply hydrogen-rich reformate to fuel cell assemblies) can over time degrade their structures to the point where the CPOX reaction proceeds in a very inefficient manner, if at all.
In the case of foam monoliths possessing a sturdier pore structure, such structure tends to magnify the randomness of the gas flow paths therethrough. While damage to the pore structure owing to hot spots can be negligible or nonexistent, the problem of scattered and fleeting cold spots that negatively affect the productivity of the CPOX reaction remains a drawback of this type of foam monolith.
It will also be noted that even when manufactured by a well-defined, closely-controlled process, foam monoliths will differ in their pore structures, and therefore in their gaseous flow properties, from other foam monoliths produced by the same process. As a result of unavoidable differences in their microstructures, individual foam monoliths produced by the same process of manufacture tend to exhibit idiosyncratic operational characteristics that can only be determined empirically. As a practical matter, a broader range of performance and reliability parameters or specifications will be assigned to reformers incorporating foam monoliths of the same manufacture in order to make allowance for the unpredictable variations in their performance.
Accordingly, the industry desires new designs of CPOX reformers and new methods of CPOX reforming that can address certain of the disadvantages of the prior art.